Tech guide — GPR antennas
GPR Antenna Frequency Guide: 200 vs 400 vs 900 MHz
How centre frequency, soil and wavelength decide how deep a radar sees and how small a target it can separate.
Quick AnswerGPR antenna frequency decides the balance between depth and detail. Around 200 MHz reaches several metres but blurs small targets. A 400 MHz antenna suits most cables and pipes under roads to about 2.5–3 m, and 900 MHz shows fine shallow detail to about 1 m. Wet clay and salty ground cut all of these. For how radar compares with a cable locator, read EM Cable Locator vs GPR.
How does GPR antenna frequency change what the radar can see?
GPR antenna frequency sets a trade-off: a lower frequency travels deeper but blurs detail, while a higher one shows fine detail and fades sooner. Ground penetrating radar sends short electromagnetic pulses into the soil and times the echoes from buried edges. A 200 MHz antenna may reach several metres in dry sand yet merge two small cables. A 900 MHz unit picks out a thin slab conduit but rarely sees past a metre, so street work on underground utilities in India mostly uses 300 to 500 MHz.
The pulse bandwidth is roughly as wide as its centre frequency. Soil absorbs the high-frequency part faster, and conductive ground such as clay, saline water or slag fill absorbs it fastest. The figure printed on the antenna is therefore a statement of how much detail you will trade for depth.
Why does wavelength in the ground matter more than the MHz figure?
Resolution depends on the wavelength inside the soil, and that wavelength shrinks as the ground gets wetter. Wave speed is v = 0.3 / √εr metres per nanosecond, where εr is the relative dielectric permittivity. Dry sand (εr 3 to 5) carries the wave at about 0.15 m/ns, while wet sand and most clays slow it to about 0.06 m/ns. Wavelength is then λ = v / f.
In a moist alluvial verge (εr near 9, v ≈ 0.1 m/ns) the 400 MHz wavelength is 0.25 m. Vertical detail is about a quarter of that, roughly 6 cm, so a duct 4 cm above another shows as one echo. Side-by-side detail follows the beam footprint: the first Fresnel zone radius, √(λd/2), at 1 m depth is about 0.35 m at 400 MHz. It grows to 0.5 m at 200 MHz and shrinks to 0.24 m at 900 MHz, so cables 30 cm apart may merge at the lower band.
Which antenna frequency suits which job?
Use the lowest frequency that still separates the targets you care about. For most street utilities that means a 400 MHz class antenna, backed by a second band where depth or detail demands it. The table assumes v ≈ 0.1 m/ns, with typical working limits in average Indian soils, not promises.
| Centre frequency | Wavelength in soil | Vertical detail (≈ λ/4) | Typical working depth | Usual jobs |
|---|---|---|---|---|
| 100 MHz | 1.0 m | ≈ 25 cm | Several metres; beyond 10 m only in dry, resistive ground | Geology, water table, deep fill |
| 200–250 MHz | 0.4–0.5 m | ≈ 10–12 cm | Up to about 5 m | Trunk sewers, large mains, deep HDD crossings |
| 400–500 MHz | 0.2–0.25 m | ≈ 5–6 cm | Up to about 2.5–3 m | Cables, ducts and pipes under roads and verges |
| 900 MHz–1 GHz | ≈ 0.1 m | ≈ 2.5–3 cm | Up to about 1 m | Shallow services, footpaths, paving |
| 1.5–2.6 GHz | ≈ 0.04–0.07 m (in concrete) | ≈ 1–2 cm | Up to about 0.5 m | Rebar, conduits and voids in slabs |
Line layout matters as much as frequency: single lines suit routes, while dense grids catch services in any direction (see: 3D GPR vs 2D GPR).
How deep can each GPR antenna frequency see in Indian ground?
Depth depends more on the ground's conductivity than on the antenna, so one 400 MHz unit may see 3 m in one district and under 1 m in the next. A US Environmental Protection Agency example puts moist conductive material (100 mS/m, εr 20) at about 38 dB of loss per metre. A 100 dB system then runs out near 2.6 m.
- Indo-Gangetic alluvium (Delhi NCR, Uttar Pradesh, Bihar): moderate reach that drops after the monsoon.
- Black cotton soil (Deccan, Gujarat, Madhya Pradesh): wet clay where even 200 MHz struggles.
- Coastal and reclaimed ground: salty pore water, often under a metre.
- Rock and laterite (Aravalli ridge, the south): good reach, with scatter at fractures.
Depth readings also rest on the wave speed chosen during processing; for how that speed is calibrated on site, see: GPR Depth Accuracy.
How do we choose the frequency on a real job?
We start from the deepest and smallest target in the brief, judge the ground, and prove the choice on a short test line before the crew commits to the route.
- Brief and existing records: an HDD crossing at 3 m needs a different set-up from a footpath duct at 0.6 m.
- Ground: soil, recent rain, fill and road crust all change the loss.
- Window and spacing: 3 m at 0.1 m/ns needs 60 ns of two-way time, so we record about 80 ns. Traces sit no wider apart than a quarter wavelength.
- Test line: a scan across a known manhole connection compares the bands before the full route.
Our radargram pages show each band on real ground; there is more on the GPR Radargram Library for comparing images side by side.
No antenna sees through metal or tells which of several identical arches is the live cable. Concrete pipes and lone optical fibres can also return faint echoes, as US Federal Highway Administration guidance notes. Frequency choice therefore sits inside a wider method that helps reduce risks on construction projects: a locator traces conductive lines, and vacuum excavation confirms a critical mark.
Grading each line by quality level is what a SUE survey, in short, adds, and the resulting utility maps help in avoiding conflicts and accidental utility strikes.
What does a GPR survey cost with the right antenna set-up?
The rate depends on the drawing, not the antenna: ₹24 per metre for standard scanning and ₹32 per metre with an L-section, GST extra. The smallest order we accept is ₹50,000 within Delhi NCR, or ₹1,00,000 elsewhere in India. Long routes earn a discount on the whole order of 25% above 100 km, 40% above 500 km and 50% above 1,000 km. Inside NCR, a crew can scan on the day you book or the next working day. In other cities the scan is run by our operator based there, starting immediately or inside a single working day. Where mobilisation or travel applies, it is a line in the quote.
Tell us the deepest service you expect and the ground conditions, and we will propose the antenna set-up with the quotation; share your project details to start.
Sources
- US Environmental Protection Agency, “Ground Penetrating Radar”, Environmental Geophysics archive (velocity, attenuation and dielectric table). archive.epa.gov
- US Federal Highway Administration, InfoTechnology, “Underground Utilities – Ground-Penetrating Radar (GPR)”. infotechnology.fhwa.dot.gov
- ASTM International, D6432-19, “Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation”. store.astm.org
- University of British Columbia, EOSC 350 “Geophysics for Practising Geoscientists”, GPR notes (velocities and survey design). www.eoas.ubc.ca
Frequently asked questions
Is a higher GPR frequency always better?
No, higher frequencies give sharper detail but lose energy faster. On wet or clayey ground a 900 MHz antenna may see almost nothing below 50 cm while a 400 MHz unit still shows the services.
What frequency is used for utility mapping in India?
Most street work uses a 300 to 500 MHz class antenna, with a lower band for deep crossings and a higher one for crowded footpaths.
Can one antenna cover both deep and shallow targets?
A dual-frequency antenna records a low and a high band in the same pass, which covers many jobs. Targets far deeper still need a lower-frequency unit.
Why does the same antenna reach less after the monsoon?
Moisture and dissolved salts raise permittivity and conductivity, so the wave slows and loses energy until the ground dries.
What does "centre frequency" mean on a GPR antenna?
It is the middle of the transmitted band, so a 400 MHz antenna covers about 200 to 600 MHz. There is more about this in the GPR, SUE & Land Survey Glossary.
Which frequency is used for concrete scanning?
A 1.5 to 2.6 GHz antenna, which resolves bars and conduits within the top half metre of a slab.
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GPR methods
Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026